In the automatic testing of integrated circuits (IC) and other electronic devices, it is desirable to bring the device to the proper temperature and to place the device to be tested in position. To perform these steps, an apparatus which is referred to as a device handler may be used. A device handler may include a wafer prober or any other apparatus that might hold the device being tested. The electronic testing itself is provided by a large and expensive automatic testing system, which includes a test head, which has been required to connect to and dock with the device handler. In such testing systems, the test head has been usually very heavy—on the order of 40 to 1000 (or more) kilograms. The reason for this heaviness is because a test head is densely packaged with electronic circuits in order to achieve accurate high speed testing.
Test head positioner systems may be used to position the test head with respect to the device handler. When the test head is accurately in position with respect to the device handler, the test head and the device handler are said to be aligned. When the test head and device handler are aligned, the fragile test head and device handler electrical connectors can be brought together (i.e. docked), enabling the transfer of test signals between the test head and the device handler. Thus, before being brought together, the fragile test head and device handler electrical connectors must be precisely aligned to avoid damaging the fragile electrical connectors.
Test head positioners are designed in several configurations, each configuration being desirable for a particular purpose. Many positioners include a test head “mounting unit.” The mounting unit supports the test head and may provide one or more axes of motion for the test head. The test head mounting unit may comprise a “pivot cradle,” a “translation cradle”, a “yoke,” or other apparatus. Generally, a pivot cradle, a translation cradle, and a yoke all comprise two parallel structures, which are arranged next to the two opposite sides of the test head and to which the test head is attached. With a pivot cradle the test head is mounted in such a way that it can pivot about an axis, which is generally orthogonal to the two parallel structures. With a translation cradle the test head is mounted in such a way that it can slide in and out, and possibly also pivot. With a yoke, the test head is rigidly attached to the two parallel structures. Examples of other structures are mentioned later. Hereinafter, the term “cradle” is used to mean a pivot cradle, a translation cradle, or a yoke.
In a tumble mode positioner, for example, the mounting unit is a pivot cradle; the test head pivots (or tumbles) about two oppositely disposed pivot points within the pivot cradle. This enables the user to tumble the test head in the pivot cradle from a position where the device handler interface board is up (for interface to horizontal plane handlers from the bottom), through 180 degrees or more, to a position where the device handler interface board is down (for interface to horizontal plane handlers from the top). An example of a tumble mode positioner is disclosed in a previous patent by Smith (U.S. Pat. No. 4,705,447), herein incorporated by reference.
In a cable pivot mode positioner, the test head pivots on the axis of the test head cables. Compared to the tumble mode positioner, a cable pivot mode positioner allows the use of reduced cable lengths. In many cable pivot positioners, the test head is rigidly attached to a yoke. The combination of yoke and test head are attached to the positioner in a manner that allows them to be rotated through 180 degrees or more about the axis of the test head cables for the previously mentioned purposes. There are several ways of implementing cable pivot positioners as described for example in U.S. Pat. Nos. 5,900,737, 5,608,334, 5,450,766, 5,241,870, 5,030,869, and 4,893,074.
Also, in both tumble mode and cable pivot positioners, it is typical to provide a means to allow the test head to be pivoted at least a few degrees about an axis that is orthogonal to the axis that provides the 180 degrees of rotation and that is parallel to the device handler interface. Thus the test head mounting unit may provide both pitch and roll motions for the test head.
Still other positioners incorporate translation cradle mounting units which provide translational in-out motion in addition to one or more axes of pivotal motion. The aforementioned U.S. Pat. Nos. 5,241,870 and 5,450,766 provide examples of such units. In these examples the test head is attached to slide units which slide in and out along two parallel structures, which are arranged next to the two opposite sides of the test head. Typically, the test head is attached to the slide units so that it may pivot about the axis defined by the two points of attachment.
Still further positioners do not use cradles. In one example, the test head mounting unit comprises a gimbal-like mechanism internal to the test head which provides rotational degrees of freedom to the test head. A first cable pivot axis supports the gimbal unit. The gimbal unit supports a second axis, which is orthogonal to the first cable pivot axis and parallel to the test interface board, and that directly supports the test head. The test head may pivot about the second axis.
After a test head is docked with a device handler, the test head may be maintained in an unlocked state relative to the axes about which it pivots. This may be done to allow the transmission of vibration from the device handler into the positioner system so that all of the vibration is not absorbed by the fragile electrical connectors, which could be destructive. In other words, by unlocking the axes of rotation so that the test head is in a “floating” state, vibrational forces are dissipated to the positioner system.
When a test head is situated in a positioner, it is desirable that the test head pivot within the mounting unit about the center of gravity of the test head. “Center of gravity of the test head” with respect to a pivot axis provided by the mounting unit means the center of gravity of the test head combined with the portions of the mounting units which pivot with it and cables or other equipment effecting balance of the test head within its mounting unit. While, during normal installation, the test head is attached to the mounting unit at the center of gravity of the test head, the center of gravity of the test head may change. This may occur, for example, if circuit boards are added (or removed from) the test head. Also, the center of gravity of the test head may be affected by the cables that extend into the test head. It is common for the cables to provide up to 30% of the total weight of the combination of the cables and the test head.
Components of the mounting unit may pivot with the test head in one or more axes. For example in a tumble mode system such as described in U.S. Pat. No. 4,705,447, the test head pivots with respect to the pivot cradle in a first axis, and the combination of the test head and pivot cradle pivot about an orthogonal axis. As another example in the cable pivot positioner shown in FIG. 7 of U.S. Pat. No. 5,450,766 the test head and yoke pivot as a combined unit about a first axis, and the test head yoke and other apparatus pivot as a unit about a second orthogonal axis. It is highly desirable for each pivot axis to pass through its respective center of gravity. In these examples, the portion of the mounting unit that pivots with the test head effectively becomes “part” of the test head to the extent it pivots with the test head.
If, however, the test head has not been positioned to pivot about a center of gravity, and is hence unbalanced then gravitational forces will try to urge the test head towards a balanced state. This may create a significant amount of stress on the test head pins through which signals are received from and transmitted to the device handler. As a test head may weigh 1000 kilograms (or more), if the center of gravity is offset by ⅛ inch (for example), the lateral forces which may be applied to the test head pins as a result of this imbalance may be considerable. By applying such forces to the extremely small and fragile pins, the pins may become worn or damaged. Alternatively, as the test head weight and imbalance may be supported by other structures (i.e., cams and guide pins), these other structures may also become worn or damaged by the imbalance. Thus, it is highly undesirable for the axis about which the test head pivots to be offset from its center of gravity.
In the past, the position of the test head in the mounting unit has been adjusted by such techniques as swapping in supports of different lengths (until the test head is pivoting about its center of gravity) or using a multiple pivot point pivot cradle and changing the pivot point which is used to couple with the test head. Yet another technique to adjust the location of the center of gravity is to add or remove weights or ballast to or from the test head. All such techniques are cumbersome and may require dismantling the equipment to provide the adjustment.